1. Field of the Invention
The present invention relates to an apparatus for detecting and locating faults in a two-way communication system using a single optical path according to SDH (Synchronous Digital Hierarchy) or PDH (Plesiochronous Digital Hierarchy) or others.
2. Field of the Invention
As society has grown to become more information-oriented, it has been increasingly required that communication lines have high reliability. For this reason, communication systems are desired to be essentially free from faults, but once a fault occurs it is necessary to detect the occurrence of the fault and to specify the location of the fault as soon as possible to remove the cause of the fault and restore the system.
In a two-way communication system using a single optical fiber, if a fault occurs in the optical fiber, an optical signal sent from a station is reflected at the fault position of the optical fiber and returns to the station that is waiting for an optical signal sent from a station of another party. If the station cannot distinguish the reflected signals from the signal sent from the other party, occurrence of the fault cannot be immediately detected.
In order to immediately detect the occurrence of a fault in a two-way communication system using a single optical fiber, conventionally, signals in one direction of the two-way communication are inverted in a sending side and received signals in that direction are inverted also in a receiving side. Signals are not inverted in the other direction of the two-way communication. In this construction, if a transmission signal is reflected at a fault point, a station waiting for inverted signals receives non-inverted signals and a station waiting for non-inverted signals receives inverted signals and therefore the frame synchronization is lost. Occurrence of the fault can be immediately detected by detecting the loss of frame synchronization.
In this construction, however, a loopback test for locating a fault cannot be executed, because frame synchronization is not established even when normal signals have returned back to receiving side.